Hitherto, an α+β titanium alloy has been used as an aircraft member by utilizing its high specific strength. In recent years, the weight ratio of a titanium alloy to be used in aircraft members is increasing, and this alloy may become more and more important. In addition, for example, also in the consumer products field, an α+β titanium alloy which is characterized by high Young's modulus and light specific gravity thereof, may be often used for the application to a golf club face, etc.
Further, the high-strength α+β titanium alloy may be expected to find its future application in an automotive component wherein a reduction in the weight thereof is important, in a geothermal well casing requiring corrosion resistance and specific strength, and the like. In particular, the titanium alloy may be used in the form of a sheet in many cases, and therefore, the needs for high-strength α+β titanium alloy sheet may be high.
As the α+β titanium alloy, Ti-6% Al-4% V alloy (herein, “%” is mass %, in the same manner hereinafter) may be most widely used and a representative alloy, but it has poor hot workability. When the α+β titanium alloy is subjected to hot rolling, edge cracking, that is, cracking along the sheet width direction, may be generated in both edge parts of the resultant hot-rolled sheet.
When a hot-rolled coil with edge cracking remaining is intended to be cold recoiled or uncoiled for such as tension leveling or the like, there may be a posed problem such that a crack may propagate in the sheet width direction starting from the edge cracking, to thereby cause a sheet fracture (or a fracture through the width direction of the sheet) in some cases. In other words, the α+β titanium alloy may have a drawback that cold coil handling property thereof is poor.
When the sheet fracture occurs, the fractured sheet must be removed from the production line, and the production may be inhibited because of the reason, for example, that the removal thereof takes time. Accordingly, the production efficiency may be reduced and at the same time, a safety problem may arise, for example, such that the sheet itself or a piece of the fractured sheet may come to fly suddenly due to the impact upon the fracturing.
Further, the sheet may significantly be deformed near a portion where the fracture has occurred in the sheet, and the portion cannot be used as a product in many cases. As a result, the production yield may be dropped, and the coil may be reduced in the unit mass, so as to further decrease the production efficiency and yield.
In this case, it may be a most effective solution that the coil is trimmed in a slitting step so as to remove the edge cracking generated in the hot-rolled coil, and then subjected to a cold leveling step. However, when the tension that works on the sheet during cold leveling is fluctuated due to plugging with trimming scraps during the trimming, sheet fracture may occur. Also, when the edge cracking is deep, the reduction in the production yield due to the trimming may be high, so as to cause an increase in the production cost.
For these reasons, there has been demanded, mainly, an α+β titanium alloy hot-rolled sheet such that it has a superior handling property, ensures that a crack initiating from edge cracking may hardly be propagated in the width direction of the sheet, and also is excellent in cold recoiling and/or uncoiling property, and it can produce a cold rolled strip. To meet this demand, several α+β titanium hot-rolled alloys capable of producing a cold-rolled strip have been proposed.
Patent Documents 1 and 2 propose an α+β titanium hot-rolled alloy of low-alloy system containing Fe, O and N as main alloying elements. This titanium hot-rolled alloy may be an alloy where Fe is added as a β stabilizing element and inexpensive elements, O and N, are added as α stabilizing elements in proper ranges at a proper balance, so as to achieve a high strength-ductility balance. In addition, the titanium hot-rolled alloy mentioned above has a high ductility at room temperature and therefore, it may also be an alloy capable of producing a cold-rolled product.
Patent Document 3 discloses a technique where Al capable of contributing to the achievement of high strength but of decreasing ductility so as to reduce the cold workability is added, and, on the other hand, Si or C which is effective in increasing the strength, but does not deteriorate the cold rollability is added, to thereby enable the cold rolling. Each of Patent Documents 4 to 8 discloses a technique for enhancing mechanical characteristics by adding Fe and O and controlling the crystal orientation, crystal grain size or the like.
Patent Document 9 discloses a technique where in pure titanium, the grain is refined and hot rolling is started in β single phase region so as to prevent the generation of wrinkles or scratches. Patent Document 10 discloses an α+β casting titanium alloy of Ti—Fe—Al—O system for a golf club head. Patent Document 11 discloses an α+β titanium alloy of Ti—Fe—Al system.
Patent Document 12 discloses a titanium alloy for a golf club head, where the Young's modulus is controlled by a final finish heat treatment. Non-Patent Document 1 discloses a technique for forming a texture in pure titanium through heating to β region and subsequent uni-directional rolling in the α region.
However, these techniques are not to enable cold coil handling property of a hot-rolled sheet by controlling the structure of an α+β titanium alloy hot-rolled sheet to thereby enhance the toughness of the hot-rolled sheet.